The present inventions relates to a semiconductor device, a semiconductor packaging method and a method for fabricating an assembly, and more particularly, it relates to a BGA (ball grid array) type semiconductor device in which a plurality of ball electrodes are two-dimensionally arranged.
Solder joint is indispensable in a semiconductor device and a package substrate using the semiconductor device. Even today, a tin-lead eutectic solder is widely used. The tin-lead eutectic solder is used for the solder joint for the following reasons:
First, the tin-lead eutectic solder has a low eutectic point. The tin-lead eutectic solder reaches its melting point at a temperature lower than the heat resistant temperature of not only a semiconductor device itself but also a printed board used in the substrate packaging. Therefore, when the tin-lead eutectic solder is used, the solder joint can be obtained without thermally damaging the semiconductor device and the printed board. In addition, the tin-lead eutectic solder has high soldering reliability. In order to secure the definite operation of the device, high soldering reliability is significant.
However, in consideration of terrestrial environmental problems currently discussed, products including lead are now reconsidered to be replaced. Specifically, lead is eluted from such products owing to acid rain and pollutes ground water, and drinking of this polluted water is said to cause blood disorder or central nervous disorder in a human body. Therefore, a solder not including lead is attracting attention.
A solder not including lead (hereinafter referred to as the “lead-free solder”) has been earnestly studied these several years, and a substantially limitless kinds of solders resulting from combinations of a variety of components and their mixing ratios have been developed. Accordingly, the composition of a solder used by a manufacturer of semiconductor devices, that is, a supplier of semiconductor devices, is occasionally different from the composition of a solder used by a set assembler, that is, a receiver of the semiconductor devices.
Even when the compositions of the used solders are different, if the amount of the solder used on a semiconductor device is very small and much smaller than the amount of a solder cream used on a set substrate as in, for example, electroplating, it is said that the different compositions of the solders minimally cause harmful influence. However, in the case of, for example, a ball grid array package (hereinafter referred to as the “BGA package”) having a solder ball electrode, the mass and the volume of the solder ball electrode are substantially the same as those of the cream solder used on a package substrate. Therefore, the resultant package seems to be largely influenced by the different compositions of the solders. Specifically, since the different solders have different melting points, air contamination is caused between the solder electrode and the cream solder on the package substrate during the solder joint of the BGA package performed in a reflow furnace, and the contamination leads to voids, resulting in lowering the soldering reliability.
At present, with respect to the lead-free solders, various studies and examinations are made on Sn—Ag—Cu-based solders in particular, and Sn—Ag—Cu—Bi-based solders are also studied and examined. Also, a cream solder made of such a lead-free solder has been put to practical use, and a solder metal ball made of a Sn—Ag—Cu-based solder has been developed. On the other hand, apart from the Sn—Ag—Cu-based solders and Sn—Ag—Cu—Bi-based solders earnestly studied and examined, examinations are being made also on Sn—Zn-based solders, which have an advantage of a comparatively low melting point although they are disadvantageously easily oxidized, and a cream solder made of a Sn—Zn-based solder has been developed. However, a solder metal ball electrode made of a Sn—Zn-based solder has not been realized yet.